The present invention relates to x-ray imaging. In particular, x-rays for diagnosis or therapy are detected.
Image guided radiation therapy uses portal imaging. The treatment x-rays create a portal image. Typical therapeutic x-rays are high energy, such as generated by a 6-25 Megavoltage source. Such high energy x-rays are not optimum for imaging. The high energy photons have a high rate of penetration. The high rate of penetration reduces the number of photons detected. Due to the poor quantum efficiency, a low signal or contrast to noise ratio is provided.
Flat panel detectors are typically used in portal imaging. High energy x-ray photons interact with a phosphor screen to generate photons and energies visible to the detector, such as visible light. Flat panel detectors detect the converted energies. To increase detection efficiency, a build-up plate of copper or other material is usually placed between the source of the x-rays and the phosphor screen. The build-up plate converts some of the x-ray photons into secondary electrons. The secondary electrons interact with the phosphor screen to generate additional light. Even with a build-up plate, a high percentage (e.g. about 99%) of the high energy x-ray photons pass through the flat panel detector without being detected. A greater efficiency is provided by increasing the thickness of the phosphor screen. A thicker phosphor screen provides more opportunity for the high energy x-rays to interact with the phosphor screen to generate additional light photons. However, a thicker phosphor screen reduces the spatial frequency response. Interactions spaced further away from the detection circuitry in the phosphor screen are mere widely disbursed, reducing the modulation transfer function.